Hitherto, the maintenance inside a nozzle stub of a reactor vessel has been performed under water since the inside of the reactor vessel is generally filled with cooling water. However, in order to perform the maintenance under water, all devices need to be designed to be waterproof. For this reason, since the devices are designed to be waterproof, the volume and the weight of the devices increase, so that the handling of the devices is difficult. Further, since the used device is exposed to radiation contained in the cooling water, the device needs to be decontaminated after it is lifted. For this reason, recently, a method of performing the maintenance inside the nozzle stub of the reactor vessel in atmosphere has been proposed.
Specifically, a platform is installed inside the reactor vessel after the water level of the cooling water therein is made to be lower than that of the nozzle stub. Inside the platform, a workman inserts a UT inspecting device into the nozzle stub to be fixed therein, and performs UT inspection in a desired range.